Ask HN: What are these strange files that appeared on a WordPress website?

I noticed the following very large files that appeared on my server on a specific domain:

core.926

core.930

core.957

core.958

core.959

core.961

Each of the files is about 5GB in size. The web developers working on the website have no idea how the files got there.

I've since deleted them, but I'm just curious if anyone knows what these are. Is this some sort of vulnerability? Malware? Out of date WP plugins causing this? A search doesn't seem to pull up anything useful.